One of the challenges of all sensing systems is to be able to operate in a number of hostile environments. Intrusion detection systems, which are often installed in outdoor environments and need to operate during periods of heavy wind or rain, or close to nearby traffic crossings, are no exception.
In any sensing system, a nuisance alarm can be defined as an alarm caused by an event that is not of interest for that sensing system. For intrusion detection systems, this relates to non-intrusion events such as wind, rain, vehicular traffic and other environmentally related non-intrusion events. Nuisance alarms can adversely affect the performance of intrusion detection systems, as well as the confidence of the system operator. The minimization of the nuisance alarm rate of intrusion detection systems, and indeed of any sensing system, is therefore critical for its successful performance and confidence of operation.
An important part of nuisance alarm handling involves being able to recognize the nuisance event being detected by the sensing system, as well as being able to discriminate between nuisance events and intrusion events. A number of different signal processing techniques can be used to achieve this and can range from simple filtering techniques, to adaptive filtering techniques, to a number of time-frequency analyses. The crux of all event recognition and discrimination techniques is the signal classification process, which involves extracting and identifying unique features in event signals. The event signals may represent isolated individual events (for example intrusion, rain, wind or traffic), or a number of events occurring simultaneously (for example, an intrusion event during heavy rain). In this latter case of simultaneously occurring events, an effective technique for extracting the event of interest from the event of non-interest is required.
In some instances it is also desirable to be able to classify the particular type of nuisance event.
The intrusion detection system may be of the type described in U.S. Pat. Nos. 6,621,947 and 6,778,717, and U.S. patent application Ser. No. 11/311,009. It is based on a bidirectional Mach Zehnder (MZ) which can be used as a distributed sensor to detect and locate a perturbation anywhere along its sensing arms. It will be referred to as a locator sensor. The content of these patents and the application are incorporated into this specification by this reference.